This invention relates to signal processing, and more particularly it relates to the resolution of errors caused by analog and digital impairment of telephone networks during the signal transmission between high speed modems.
Telephone networks are generally used for voice as well as data communications with reference to data communications, a plurality of high speed modems (one of which is typically remotely disposed at the premises of a customer) are coupled together via a telephone network. Each modem is typically connected to the telephone network by a local exchange carrier (LEC) subscriber loop.
Generally, telephone networks include both analog and digital components. The digital components may include a T1 digital link, an integrated service digital network (ISDN), a fiber optic cable network, a coaxial cable network, a satellite network, and/or a wireless digital communications network. Further, the LEC subscriber loop may comprise either an analog or a digital communications path.
During a data communication session between remotely disposed modems, one modem (the transmitting modem) initiates a data transmission and another modem (the receiving modem) receives the transmission. Usually an A/D converter is required for converting analog signals from an analog portion of the network into digital signals for subsequent transmission over the digital portion of the network. Similarly, a D/A converter is required for converting digital signals carried on the digital portion of the network into analog signals for subsequent transmission over the analog portions of the network.
In North America and Japan, data communications over the telephone networks are conducted in accordance with pulse code modulation (PCM) xe2x80x9cxcexc-lawxe2x80x9d encoding and decoding techniques. In Europe, communications are conducted in accordance with xe2x80x9cA-lawxe2x80x9d encoding and decoding technique. In accordance with those well-known PCM xcexc-law or A-law techniques, signals are transmitted in the form of PCM signals. PCM signals consist, in general, of a series of binary code words in which each word represents an instantaneous value of a periodically sampled and quantized analog signal. In normal usage, these code words are transmitted in the form of a serial bit stream to a receiving subscriber loop where they are decoded into a reconstructed version of the original analog signal.
When a PCM code goes through a telephone network, the PCM code values can be changed due to digital switching in the telephone networks. These changes introduce an error element known as xe2x80x9cimpairmentxe2x80x9d which can have both digital and analog components. The combined analog and digital components are known collectively as xe2x80x9cnetwork impairment.xe2x80x9d The network impairment causes PCM modems to mis-code signals and interferes with the performance of the data communication.
Attempts have been made to compensate for network impairment. These techniques include the use of encoders/decoders which hold digital impairment tables depicting preselected analog levels corresponding to the digital code words in accordance with the xcexc-law or A-law standards.
FIG. 4 illustrates a prior-art telephone network configuration 10 implementing one of these prior-art techniques. In FIG. 4, two remotely disposed (client site) modems 101 and 115 are coupled via a typical telephone network. In the outgoing (transmitting) direction, modem 101 transmits a signal to a first hybrid 103. The hybrid 103 separates a bidirectional analog signal received from the modem 101 into a unidirectional transmit signal. This unidirectional transmit signal is then converted to a digital signal at A/D converter 105. The digital signal is then encoded into a PCM signal at encoder 107, which routes the encoded signal to a digital network 113. After traversing the digital network 113, the encoded signal is decoded at decoder 109 and is then converted back (regenerated) to an analog signal at D/A converter 111. The analog signal is then output to a second hybrid 103A which separates the bidirectional signal received from the D/A converter 111 into a unidirectional signal and delivers the unidirectional signal to the second modem 115.
In the reverse incoming (receiving) direction, the signal follows an essentially identical path in the reverse direction via the second hybrid 103A, A/D converter 105A, encoder 107A, digital network 113, decoder 109A, D/A converter 111A, and the first hybrid 103.
In the configuration of FIG. 4, a typical signal is encoded/decoded many times while being transmitted through the telephone network 10. The constant encoding/decoding and conversion between digital and analog signals produces an inherent quantization error in the transmitted signal. This error arises from the fact that the amplitude of the regenerated analog signal does not exactly match the analog signal level of the original signal.
FIG. 5 illustrates an alternative telephone network configuration 20 in which the A/D or D/A conversion and the need for encoding/decoding between digital and analog signals is reduced. This is a typical configuration of a communications network wherein a central site modem 201 (a network modem) downloads information to a client modem 210 (for example, an internet application where a network modem downloads large blocks of information to client site modem 210). In the configuration of FIG. 5, central site mode 201 is coupled directly to the digital network 213. The quantization error is reduced because there is no conversion or encoding/decoding between the modem 201 and the digital network 213. The connection between the digital network 213 and the client site modem 210 is essentially identical to the same connection in FIG. 4.
U.S. Pat. No. 5,724,393 (the ""393 patent), assigned to the assignee hereof, discloses a method of reducing the quantization error resulting from the encoding/decoding in transmissions between modems. As explained in the ""393 patent, the quantization error can be reduced by scaling down the amplitude levels of signals generated by the transmitting modem by a predetermined reduction factor and then performing an inversion mapping in which the amplitude level of the factored, xcexc-law encoded signals is scaled up by a predetermined inversion factor.
Typical prior art systems for reducing telephone network impairment rely on the use of predefined impairment tables associated with the most commonly used telephone switches. The need to use large predefined telephone network impairment tables reduces the flexibility of these prior art systems and increases the cost. In addition, this solution can be difficult to deploy in a long distance telephone network which combines different types of switches (e.g., mixed A-law and xcexc-law standards) because the encoders and decoders used in the system will vary system-wide depending on which standard is in use.
Thus, there exists a need for a general, low cost method for detecting and compensating telephone network impairment which may result in a constant signal performance over a wide dynamic range (with minimum quantization error) and which may be applicable to a variety of telephone networks including mixed xcexc-law and A-law networks.
A method and apparatus for automatically detecting and compensating for telephone network impairment in a telephone network is disclosed. The inventive method eliminates the need for predefined digital impairment tables and can be applied to any telephone network including those which utilize different types of switches and xcexc-law/A-law mixed switches. The inventive method also eliminates the quantization error typically present in the transmission of signals due to the use of encoders/decoders. It is a low cost solution and does not require any pre-knowledge about the telephone network and may be used in telephone network deploying switches which are currently under development or often upgraded.
According to the present invention, a first modem (acting as a transmitting modem), coupled to a second modem (acting as a receiving modem) by a telephone network, computes analog and digital impairment before establishing an actual data transmission session. The transmitting modem (typically a digital modem) sends a digital test signal to the receiving modem. The received test signal is analyzed and total network impairment (analog and digital combined) in the telephone network is determined by comparing the transmitted test signal to the received test signal and identifying any differences (for example miscoding). The received signal has an analog component and a digital component.
Next, the analog impairment is computed separately. To compute the analog impairment, first a corresponding analog level of the transmitted digital test signal is computed or estimated in accordance with xcexc-law encoding/decoding techniques or A-law encoding/decoding techniques. After a corresponding analog level is known, analog impairment is computed by comparing the corresponding analog level to the analog component of the received test signal. The difference between the analog level and the received test signal is the analog impairment (also known as distortion). The digital impairment is then calculated by comparing the total telephone network impairment and analog impairment and finding a difference between them.
After analog impairment and digital impairments have been computed, the appropriate compensation for analog impairment and digital impairment is performed by using prior art compensation schemes. There are separate compensation schemes for analog and digital impairments and these compensation schemes negate the effects of telephone network impairment.
The actual data transmission from transmitting modem to receiving modem occurs with the selected compensation scheme being applied so that the received signal is not impaired. This ensures a constant signal transmission between the transmitting modem and receiving modem.